White sugar milled to a fine, soft powder, usually with a little starch mixed in to stop it clumping. It is a baking ingredient, used for icing and frosting and sifted over cakes and buns.
糖粉 is finer than 砂糖 shātáng (granulated sugar) and is dusted on baked goods or whipped into glazes. Don't confuse with 糖霜 tángshuāng (royal icing / sugar glaze), which is the finished frosting made from 糖粉.